The present invention relates to a device for measuring an aberration, to imaging systems comprising such a device, and to a method for measuring an aberration. The present invention particularly relates to arrangements and methods for measuring aberrations by means of optical wavefront encoding.
When light propagates through the atmosphere starting from an object, turbulence results in a spatial and temporal variation of the optical path towards a target. The result of this is that when passively observing the object or when actively illuminating the object by means of an optical system, the wavefront is deformed, which is referred to as aberration. The imaging quality and/or illuminating quality is/are decreased by aberrations. The imaging quality and/or illuminating quality may be quantified by means of the magnitude of the Strehl ratio S. Thus, a value of S=1 corresponds to ideal imaging (with limited diffraction). Mechanical deformations, like vibrations or weight-induced deformations, or thermal deformations within the optical system, and classical aberrations, like a spherical aberration, astigmatism, coma or the like, are of equivalent influence for the imaging/illuminating quality. By means of active optical elements, like a tilting mirror or deformable mirror, it is possible in principle to compensate said influence at least partly in order in increase the imaging/illuminating quality or decrease the influence of the aberrations of the wavefront. However, measuring the spatial and temporal distribution of the wavefront at the observing/illuminating system may be involved here.
Approaches for wavefront/aberration measurements are described in EP 1 983 318 A1, for example. Measuring the wavefront is done by means of a Shack-Hartmann sensor or a plenoptic camera. The aberration of the wavefront in the pupil or lens of the optical system is measured by means of a micro lens array. Thus, the micro lens array generates several, spatially separate imagings of an observed object. The relative position of the multiple imaged object features relative to one another provide information on the wavefront. However, this approach uses a point-light source for the imaging object in order to measure the wavefront However, this cannot be realized in many application scenarios. Alternatively, an extensive object may also be used, however certain object details are identified here in order to numerically reconstruct the wavefront. Due to the very low resolution (object scanning) due to the extremely small focal length of the micro lens array, identification is possible only to a very limited extent, and in many application scenarios, not at all.
Another approach described in U.S. Pat. No. 4,696,573 A suggests measuring the wavefront by means of a Shearing interferometer. A point-light source is observed here. Interference patterns of the wavefront are examined in order to reconstruct the wavefront and, thus, the aberration. However, a point-light source is used here for the object/target.
Another approach described in U.S. Pat. No. 8,907,260 B2 suggests using autocorrelation. The object distribution is measured here at different points in time. A tip/tilt aberration can be measured by means of autocorrelation of the images measured. However, this concept is of disadvantage in that several pictures are taken. The aberrations can be measured only relatively in relation to one of the pictures. Since in particular atmospheric aberrations are subject to a high change rate, disadvantages result.
Another approach described in U.S. Pat. No. 7,268,937 B1 relates to a holographic wavefront sensor. Here, the super positioning of a reference wavefront with a second wavefront which represents a certain aberration, like a Zernike mode, as a holographic optical element (HOE) is captured. When said HOE is introduced into the optical path of a wavefront to be examined, the contribution of the wavefront which corresponds to the same Zernike mode of the picture, is diffracted in two opposite orders. The relative intensity of the diffraction orders provides information on the intensity/amplitudes of the aberration. However, this concept is of disadvantage in that a point-light source is used for the object/target. In addition, the concept is of disadvantage in that it may be applied only for a narrow-band wave length spectrum.
Another approach described in U.S. Pat. No. 7,554,672 B2 relates to considering phase diversity and a curved sensor. The target here is observed at different focal settings and is observed in different diffraction orders of a diffractive element, as is described, for example, in WO 2009/058747 A1, U.S. Pat. No. 7,554,672 B2, WO 2004/113856 A1, U.S. Pat. No. 8,517,535 B2 or U.S. Pat. No. 7,531,774 B2. The wavefront aberration can be reconstructed from the different image distributions. This concept is of disadvantage in that either several optical imaging systems and image sensors are present or the images are taken in a temporally offset manner, resulting in reduced measuring speed. In addition, the concept is of disadvantage in that the reconstruction is numerically highly complicated for extensive object distributions and, thus, the measuring speed is also limited.
Consequently, a concept which allows considering or correcting aberrations also for extensive objects having little object details at high a measuring speed would be desirable.
Consequently, the object underlying the present invention is providing a device for measuring an aberration which is able to provide information relating to the aberration at high a measuring speed for both small (point) sources and also extensive objects exhibiting only a few details (“features”).